1. Field of the Invention
The invention relates to an intake control valve for a screw-type compressor, in particular for a screw-type compressor with oil injection, which is built into the intake line of the screw-type compressor and includes a locking body which alternatingly opens or closes the intake line and is operationally connected to a control piston or a control diaphragm which can be displaced in a cylinder and which can be pressurized with a pressure medium in the opening direction of the intake line.
2. The Prior Art
Various designs of such an intake control valve are known. The simple models of screw-type compressors are "normally open," i.e., in the pressureless state of the screw-type compressor the intake control valve releases the cross-sectional area of passage of the intake line. Thus, the screw-type compressor starts with an open intake control valve, thus immediately loading the driving motor owing to the entire work of compression. Not until adequate operating pressure has been built up can the intake control valve be totally or partially closed in order to reduce the quantity being transported. Thus, the drawback with this design is that the driving motor is heavily loaded and with frequent starts in the unit of time is also overloaded.
Overloading the driving motor when starting is avoided by means of the other prior art intake control valves of the "normally closed" type. In this known design the intake control valve is held closed by means or spring force. After starting the screw-type compressor, the pressure builds up significantly more slowly and correspondingly there is a smaller load on the driving motor. To accelerate pressure buildup, it has also already been known to design the closed intake control valve so as to leak, e.g., by means of a bore or a channel bypassing the valve.
As soon as a specific operating pressure is reached, the spring force holding the intake control valve closed has to be overcome in this prior art design, a process that usually takes place by means of a differential piston. The piston area of the differential piston has to be correspondingly large, because the force exerted by the piston has to overcome, on the one hand, the spring force acting on the intake control valve and, on the other hand, the gas pressure acting on the intake control valve in the closing direction by means of the built up operating pressure, additionally also the increase in the spring force as a consequence of the spring rate. Thus, it involves relatively large forces, because to guarantee a faultless function the spring force must be positively greater than the intake force of the screw-type compressor and the gas pressure must be positively greater than the spring force.
An intake controlling device of this kind is known from DE-OS 29 44 053. This device has a relatively complicated locking body which controls the intake line and which in the closing direction is spring-loaded and in the opening direction is pressurized by the pressure generated by the screw-type compressor. When the screw-type compressor is started, the spring force is overcome by the pressure building up and the intake line is opened. The cross-section of the direct flow is changed to control the amount to be transported with the aid of another control piston, which is pressurized by a control pressure, e.g., the network pressure and loads the closing body in the direction of the spring force, so that the operating pressure applied to the opposing side of the closing body is overcome.
Common to all of these known intake control valves is that they close against the direction of flow of the gas that has been taken in and have to be held closed against the intake-sided negative pressure. The result is automatically a complicated construction, which is associated with high procurement costs and can cause breakdowns. In addition, the closing spring of the locking body, locking the intake line, causes pressure losses, thus reducing the possible maximum delivery of the screw-type compressor.